Alberta Precision Laboratories

--- Query Results ---

Organization:

Alberta Precision Laboratories

Message Alert:

PLEASE NOTE: This Test Directory is in an interim state and due to changes pertaining to the DynaLIFE transition, performing site for routing has not been updated. Please refer to Laboratory Information System for appropriate routing.

Test Name/Synonym:

Phenytoin Level, Free (Synonym: Dilantin, PHENY FREE)

Clinical Indications:

Phenytoin is commonly used to treat several types of seizures and, occasionally, ventricular arrhythmias. Typically about 90% of the total phenytoin in the blood is bound to proteins, primarily albumin. Only the unbound ("free") drug is pharmacologically active. The presence of other drugs and several clinical conditions (e.g. hypoalbuminemia, uremia, pregnancy, burns and liver failure) can affect the extent of protein binding, thus altering the free drug concentration without changing the total drug concentration. Order if decreased plasma protein binding is suspected (e.g. low albumin, high creatinine, high bilirubin). Phenytoin (PTN) must also be ordered to determine % free. Phenytoin reaches steady state 1-5 weeks post dose adjustment. Free levels, like total levels, should not be ordered daily or even every second day.

Test Includes:

Phenytoin Level, Free
Phenytoin Level, Total
% Free Phenytoin

Ordering Alert:

Record time of last dose on requisition or in electronic ordering system.

Collection Alert:

For oral therapy, collection time should be consistent for a given patient (Trough levels are not imperative because of long half-life). For IV therapy, collect greater than 2 hours after end of dose.

Best practice is to not draw from an intravenous line through which the drug has been administered as it can produce erroneous results and impact patient care. If best practice is not possible, prior consultation with the most responsible health practitioner is required.

LABID (Connect Care):

LAB32

Test Code:

NORTH ZONE	PTNF
SOUTH ZONE	PTNF1

Primary Container:

Calgary: MINT LIHEP PST
Edmonton: GOLD SST

Alternate Containers:

Calgary: GOLD SST
Edmonton: MINT LIHEP PST

Minimum Collection Volume:

1.5 mL serum

Test Resources:

Half-life and Time to Steady State

Calgary Zone: Microcollection Minimum Whole Blood Volumes

Calgary Zone: Critical Values

Transportation:

Calgary: Transport at room temperature.

Method:

Kinetic Interaction of Microparticles in Solution (KIMS) assay

Testing Schedule:

Daily

Reference Interval:

Target range: 4.0 - 8.0 µmol/L based on 90% protein binding

Critical Value:

Greater than 12 µmol/L

Performing Site:

Calgary: Foothills Medical Centre
Edmonton: University of Alberta Hospital

Additional Test Information:

Steady state: 1-5 weeks (zero order kinetics)

After % free determined from initial specimen at steady state, further free phenytoin levels are not indicated if albumin and creatinine levels remain stable (therapy can be guided using total levels).
Daily free phenytoin levels are not indicated.
Free phenytoin is not performed when Total Phenytoin is Less than 10 or greater than 80.0 µmol/L.

For testing sent to Calgary:

Add on Free Phenytoin WILL NOT be performed if Total Phenytoin is already reported. Recollection for Free Phenytoin is required. Add on Free Phenytoin WILL be performed if Phenytoin has not been reported and if minimum specimen volume is available.

Last Updated On:

Wednesday, June 7, 2023